linux-kernel - Re: Expensive memory.stat + cpu.stat reads

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Message-ID: <d3f3a7bc-b181-a408-af1d-dd401c172cbf@redhat.com>
Date:   Mon, 10 Jul 2023 20:44:15 -0400
From:   Waiman Long <longman@...hat.com>
To:     Ivan Babrou <ivan@...udflare.com>,
        Shakeel Butt <shakeelb@...gle.com>
Cc:     cgroups@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        kernel-team <kernel-team@...udflare.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Expensive memory.stat + cpu.stat reads

On 7/10/23 19:21, Ivan Babrou wrote:
> On Wed, Jul 5, 2023 at 11:20 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>> On Fri, Jun 30, 2023 at 04:22:28PM -0700, Ivan Babrou wrote:
>>> Hello,
>>>
>>> We're seeing CPU load issues with cgroup stats retrieval. I made a
>>> public gist with all the details, including the repro code (which
>>> unfortunately requires heavily loaded hardware) and some flamegraphs:
>>>
>>> * https://gist.github.com/bobrik/5ba58fb75a48620a1965026ad30a0a13
>>>
>>> I'll repeat the gist of that gist here. Our repro has the following
>>> output after a warm-up run:
>>>
>>> completed:  5.17s [manual / mem-stat + cpu-stat]
>>> completed:  5.59s [manual / cpu-stat + mem-stat]
>>> completed:  0.52s [manual / mem-stat]
>>> completed:  0.04s [manual / cpu-stat]
>>>
>>> The first two lines do effectively the following:
>>>
>>> for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/memory.stat
>>> /sys/fs/cgroup/system.slice/cpu.stat > /dev/null
>>>
>>> The latter two are the same thing, but via two loops:
>>>
>>> for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/cpu.stat >
>>> /dev/null; done
>>> for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/memory.stat
>>>> /dev/null; done
>>> As you might've noticed from the output, splitting the loop into two
>>> makes the code run 10x faster. This isn't great, because most
>>> monitoring software likes to get all stats for one service before
>>> reading the stats for the next one, which maps to the slow and
>>> expensive way of doing this.
>>>
>>> We're running Linux v6.1 (the output is from v6.1.25) with no patches
>>> that touch the cgroup or mm subsystems, so you can assume vanilla
>>> kernel.
>>>
>>>  From the flamegraph it just looks like rstat flushing takes longer. I
>>> used the following flags on an AMD EPYC 7642 system (our usual pick
>>> cpu-clock was blaming spinlock irqrestore, which was questionable):
>>>
>>> perf -e cycles -g --call-graph fp -F 999 -- /tmp/repro
>>>
>>> Naturally, there are two questions that arise:
>>>
>>> * Is this expected (I guess not, but good to be sure)?
>>> * What can we do to make this better?
>>>
>>> I am happy to try out patches or to do some tracing to help understand
>>> this better.
>> Hi Ivan,
>>
>> Thanks a lot, as always, for reporting this. This is not expected and
>> should be fixed. Is the issue easy to repro or some specific workload or
>> high load/traffic is required? Can you repro this with the latest linus
>> tree? Also do you see any difference of root's cgroup.stat where this
>> issue happens vs good state?
> I'm afraid there's no easy way to reproduce. We see it from time to
> time in different locations. The one that I was looking at for the
> initial email does not reproduce it anymore:

My understanding of mem-stat and cpu-stat is that they are independent 
of each other. In theory, reading one shouldn't affect the performance 
of reading the others. Since you are doing mem-stat and cpu-stat reading 
repetitively in a loop, it is likely that all the data are in the cache 
most of the time resulting in very fast processing time. If it happens 
that the specific memory location of mem-stat and cpu-stat data are such 
that reading one will cause the other data to be flushed out of the 
cache and have to be re-read from memory again, you could see 
significant performance regression.

It is one of the possible causes, but I may be wrong.

Cheers,
Longman